Identifying Sources of Seawater Intrusion and Chlorides in Sewer Collections Systems

Seawater intrusion into sewer collection systems is a major issue for many coastal communities. Not only does it add to the total volume of water needing treatment, it also increases corrosion and effects reclaimed water quality by increasing both sodium and chloride concentrations. Recommended reclaimed water quality limits for chlorides are typically stated less than 500 mg/L (USEPA, 1992). Chloride levels are of concern in irrigation because high levels are toxic to many plants. High chlorides from seawater intrusion led the City of Punta Gorda, Florida to cease reclaimed water irrigation in 2001 (USEPA, 2004). Many utilities have adopted limits from 400 to 450 mg/L. Based on the average levels of chlorides in seawater versus freshwater, more than 65 gallons of freshwater would be required to dilute 1 gallon of seawater to 400 mg/L. This assumes chloride concentrations of 20,000 mg/L and 100 mg/L in seawater and freshwater, respectively. This may not seem significant, however during the course of a day, a 1 gallon per minute (gpm) leak would require 94,000 gallons of freshwater sewage to dilute this flow to less than 400 mg/L in the resulting reclaimed water. If the number and/or size of leaks in a collection system are significant, there may not be enough freshwater sewage to sufficiently dilute the incoming seawater and the resulting treated effluent may exceed adopted reclaimed water quality limits. The need to achieve water quality that is adequate for reclaimed water purposes is critical for many utilities along coastlines because in many cases, having a successful reclaimed water system is the primary means for disposal.